1. Field of the Invention
The present invention relates generally to nuclear fuel rods for use in nuclear reactors and, more particularly, is concerned with a system and method for collating nuclear fuel pellets so as to facilitate subsequent filling of fuel rods with pellets in multiple zones of different nuclear fuel enrichments.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and connected at opposite ends to the nozzles and a plurality of transverse support grids axially spaced along the guide thimbles. Also, each fuel assembly is composed of a multiplicity of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles.
The fuel rods each contain fissile material in the form of a plurality generally cylindrical nuclear fuel pellets maintained in a row or stack thereof in the rod. The fuel rods are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Up to the present time, generally a given fuel rod has been filled with nuclear fuel pellets of the same enrichment. In filling a typical fuel rod, pellets of the same enrichment are continually fed from supply trays into the fuel rod until the specified depth of fill has been achieved. The supply trays are used up completely. A partial tray left after completing a batch of twenty-five fuel rods is used to start the fill of the next batch. Various systems have been used in the past to fill the fuel rods with fuel pellets. Representative of the prior art are the systems disclosed in U.S. Pat. Nos. to Gerkey (4,158,601)--assigned to the assignee of the present invention--and Gheri (4,495,146), and Japanese Pat. No. 61-4999 to Mitsubishi.
Recently, nuclear fuel rods with pellets in multiple zones of different enrichments have been introduced. These new zoned fuel rods contain short lengths of "blanket" pellets at each end. The fuel rods additionally have pellets grouped within from three to five zones of different enrichments between the end zones of blanket pellets. Pellets for filling these enrichment-zoned fuel rods will first need to be assembled into the desired sequences of enrichment zones before filling of the fuel rods can commence.
One approach to assembling fuel pellets in the desired sequence of enrichment zones for filling a fuel rod is by manual effort. However, this approach envisions individual handling of each pellet which is highly labor-intensive and time-consuming and subject to human error. A much more desirable approach is to assemble the different zones of fuel pellet by use of a highly-mechanized and automated system which would be substantially free of human error.
Consequently, a need exists for an automated system capable of assembling pellets of different enrichments into the desired sequence of enrichment zones of pellets so that specified sequences of pellets will be produced for subsequent loading into fuel rods.